ADS5400: Output data at Dual Bus Mode - Aligned, CLKOUT Divide By 4

Hi,

the data format in that mode is as shown in Figure 4 of the datasheet.  On data bus B you would find sample 'N' and at that same time you would find sample 'N+1' on data bus A.  Then on the next edge of the data clock you would find sample 'N+2' on bus B right after sample 'N', and sample 'N+3' on bus A, etc.  

Since the data clock is in divide-by-four mode, if the sample rate were 1000Msps then the data clock CLKOUT would be 250MHz, and samples would be presented on the two 12-bit output busses at both rising edge and falling edge, so that each of the two sample busses would carry data at 500Msps, and together the two busses would be carrying the full 1000Msps in DDR (dual data rate) fashion utilizing both edges of the data clocks.   There are actually two copies of the data clock out, but since the two busses are aligned the CLKOUT for bus A and the CLKOUT for bus B are identical.

the thing that is most confusing about that figure4 is the attempt to show what happens after the RESET to the divide by four output clock.  The reset pulse will reset the divide by four to a known initial condition so that if there were many ADS5400 in a system and they all got their reset pulse at the same time then all the devices would have their CLKOUT in unison.    And we also imposed on ourselves a condition that if a reset pulse were to rese the CLKOUT to an initial phase position, we didn't want the CLKOUT to have a 'short' cycle when the rising edge of CLKOUT is reset to the initial condition, so if we need to we 'stretch' the CLKOUT in response to the reset pulse.   That is the meaning of the four lines labeled Phase0, Phase1, Phase2, Phase3.   If the reset pulse happens to come along at a time where it would reset the CLKOUT to a phase that happens to already be the phase CLKOUT was already in, then that is Phase 0 - no apparent change in the output of CLKOUT at all.    But if the timing of the reset pulse is such that the rising edge of CLKOUT should be delayed by 90 degrees as in the Phase1 case, then instead of a short cycle of CLKOUT we skip a pulse of CLKOUT so that there is a long cycle instead.   Likewise for the next two possible timings of the reset pulse, Phase2 and Phase3.  

But the format of the data on the two data busses is pretty simple, just one sample present on BusB at the same time as the next sample is present on BusA, as in Figure4.

Regards,

Richard P.

Hi.

Is it necessary to send RESETP after power up?
What is min frequency for RESETP?
Is it necessary to send RESETP on the rising edge of CLKINP?

As mentioned in datasheet VREF is input pin. If I set ENEXTREF = '1' or in addr 0x05h bit<2> ='1', then VREF is external and I must get to this pin is 2V. If I set ENEXTREF = '0' or in addr 0x05h bit<2> ='0', then VREF is internal? At this pin must appear output voltage = 2V?

And VCM pin. How this pin configure as output or input?

Hi,

it is not necessary to use the reset.  As described in the datasheet, the reset will reset the divider circuits that divide the sample clock down to produce the output clock in either divide by 2 or divide by 4 mode.   If you wish to synchronize multiple ADS5400 and have all of the devices operating synchronously then you would need to provide a reset pulse to all of them at the same time.  Else, reset would not be necessary.  There is no min frequency for reset, it only need happen once if you wish to use it at all, and the reset input is sampled on the rising edge of the sample clock, so if you did use reset it would have to meet setup/hold times around the sample clock.

 If you wish to use the ADS5400 in external reference mode, then you would have to provide the reference voltage of 2V to the VREF pin.   The VREF pin becomes an 'input' if you put the ADS5400 into External Reference mode.  Otherwise the VREF pin is an output that outputs the 2V reference that is generated internal to the device.  (There is a typo I believe in the data sheet in the pin descriptions for the pinout page6 where the VCM pin is described as an input/output and VREF as an input.  This should list VCM as an output and VREF as an input/output.)

so yes, as you describe if the device is put into external reference mode by either using the pin or the register bit then you would supply a 2V reference to the VREF pin.  Otherwise the device will use its internal 2V reference and you will see that 2V reference being output on the VREF pin.

One common usage for external reference is where there are multiple ADS5400 in a system and the chip-to-chip tolerance on that internally generated reference may be more than is desired for the system, and in that case the application could have an external precision 2V reference generated in the system and then that reference would be supplied to all of the data converters so that all data converters are using the same reference. 

If I understand correctly the VCM pin is should be always output. And it can be used for DC-coupled analog input. But in figure 25 show that analog inputs are internally coupled to 2.5 V. So can I use schema shown in figure 26  as AC-coupled?

Yes, the inputs are internally biased to the desired VCM voltage so that if you AC couple then you do not need to use the VCM output pin for anything.

Regards,

Richard P.

As mentioned in datasheet reference can be chanded into 2 ways: using pin programming (PPR) ENEXTREF or through serial interface addr 0x05h bit<2>. 

If i set ENEXTREF='1' and then try programming addr 0x05h bit<2>='0'. What refence will be: external or internal?

What modes has more priority: PPR or serial interface? Are changes in PPR, for example, cause changes in register map?

Hi,

As stated in the datasheet the signal from the pin for external reference is logic 'OR'-ed with the register bit for external reference.  Using either method to select external reference or both methods will result in the device being in external reference mode.  Only if neither method is used to select external reference will the device be in internal reference mode.

"Enable External Reference Mode, active high. Device uses an external voltage reference when high. This pin is logic OR'd with addr 0x05h bit<2>."  Page5.

Asserting the pin for external reference will not affect the contents of the register bit for asserting external reference. 

Regards,

Richard P.

Hi.

In addr 0x08 I can read the temperature of ADS5400 when it works (real-time measuring)?

yes, if the temp sensor is enable by setting bit D7 in register address 0x5.

regards,

Richard P.